unfortunately drag and drop file transferring/sharing is not yet implemented on virtaulbox.so we have to look for alternative method.This is what I have done.my both host and guest OS are windows , remember this is possible on Linux environment with different commands.
 Run the guest OS
open Virtualbox then select and run guest OS that you need to transfer file.Here I used Windows XP as my guest OS
 Install Guest Additions
On the VirtualBox menu click " Devices " --> " Install Guest Additions... " and follow the instruction, If you get warning similar to below Click " continue anyway" to continue the installation and finally reboot guest OS.
 Create shared folder
go Machine --> Settings... you will have Virtual machine setting windows
then select " shared Folders " ,as below
then click folder with "+" sign (right-hand corner ) to create shared folder on host OS as above screenshot.
Then you will have window called "Add share" like below, Choose the Folder Path and give a Folder Name.
Transfer file to guest OS
Start the guest OS (I used windows XP as my guest OS) open " Run " and type the below command
please note I used "sharedfiles" as folder Name. look at above screen shoot
once you type click "OK" , then you can open shared folder of the host machine.
If you reboot the guest OS this shared folder may not available therefore change shared folder setting as required
Happy file sharing !